Oral Presentation

Tracing the kpc-scale variation of the CO-to-H2 conversion factor in galaxy centers

Author(s): I-Da Chiang (ASIAA), Karin M. Sandstrom (UCSD), the z0MGS team

Presenter: I-Da Chiang (ASIAA)

The Low-J rotational lines of CO are the primary tracer for molecular gas in extragalactic studies. Our understanding of how CO brightness converts to molecular gas mass, i.e. the CO-to-H2 conversion factor, has become the factor that limits our ability to precisely study molecular ISM and star formation. We measure the variation of CO-to-H2 conversion factor in the high-surface-density regions of ~30 galaxies at 2 kpc resolution, using dust as a tracer of the total gas mass along with reasonable assumptions about the dust-to-metals ratio (D/M). We found that the CO-to-H2 conversion factor decreases with increasing stellar mass surface density, where the scaling relation is well-characterized by a power law. The slope of this power law is ~-0.4, and it is invariant of assumed D/M. Our observations suggest that at high surface densities in galaxy centers, external pressure on molecular clouds can cause increased line widths and more CO emission escape, or the molecular gas is more widespread than isolated clouds, leading to CO velocity dispersion reflecting the gravitational potential.